Download Nanopiezotronics

RESEARCH NEWS
DOI: 10.1002/adma.200602918
Nanopiezotronics**
By Zhong Lin Wang*
This article introduces the fundamental principle of nanopiezotronics,
which utilizes the coupled piezoelectric and semiconducting properties of nanowires and nanobelts for designing and fabricating electronic devices and components, such as field-effect
transistors and diodes. The physics of nanopiezotronics is based on the principle of a nanowire
nanogenerator that converts mechanical energy into electric energy. It is anticipated to have a
wide range of applications in electromechanical coupled electronics, sensing, harvesting/
recycling energy from the environment, and self-powered nanosystems.
1. Introduction
Today’s nanoelectronics rely on the accumulation and
movement of charge carriers to perform intelligence-bearing
devices, such as transistors and diodes. Spintronics is an
emerging field involving the detection and manipulation of
electron spin for sensing, data memory, and functional devices. Molecular electronics (Moltronics) utilizes molecules as
functional devices to achieve electronic performance. This article introduces a new field named nanopiezotronics,[1] which
utilizes the coupled piezoelectric and semiconducting property of nanowires and nanobelts for designing and fabricating
electronic devices such as transistors and diodes. It is anticipated to have a wide range of applications in electromechanical coupled sensors and devices, nanoscale energy conversion
for self-powered nanosystems, and harvesting/recycling of energy from environment.
Piezoelectricity (PZ) is a coupling between a material’s mechanical and electrical behavior. When a piezoelectric material is squeezed, twisted, or bent, electric charges collect on its
surfaces. Conversely, when a piezoelectric material is subjected to a voltage drop, it mechanically deforms. Many crystalline materials exhibit piezoelectric behavior, including
a
b
O
Zn c
F
+
-
P
F
Figure 1. a) Structural model of wurtzite ZnO. b) Tetrahedral coordination between Zn and oxygen. c) Distortion of the tetrahedral unit under
the compression of an external force, showing the displacement of the
center of positive charge from that of the negative charge.
–
[*] Prof. Z. L. Wang
School of Materials Science and Engineering
Georgia Institute of Technology
Atlanta, GA 30332-0245 (USA)
E-mail: [email protected]
[**] Thanks to J. H. Song, X. D. Wang, P. X. Gao, J. H. He, J. Zhou,
N. S. Xu, L. J. Chen, and J. Liu for their contributions to the work reviewed in this article. The generous support from DARPA, NSF,
NASA, and NIH is acknowledged.
Adv. Mater. 2007, 19, 889–892
quartz, wurtzite-structured crystals, Rochelle salt, lead zirconate titanate ceramics, barium titanate, and polyvinylidene
flouride (a polymer film). When such a crystal is mechanically
deformed, the positive- and negative-charge centers are displaced with respect to each other (Fig. 1). So while the overall
crystal remains electrically neutral, the difference in chargecenter displacements results in an electric polarization within
the crystal. Electric polarization resulting from mechanical
deformation is perceived as piezoelectricity.
To introduce nanopiezotronics, we first illustrate the piezoelectric behavior of a single nanowire (NW) or nanobelt
(NB)[2] of a piezoelectric material such as ZnO. For a vertical,
straight ZnO NB (Fig. 2a), the deflection of the NB by an external applied force F results in (tensile) stretching of its outer
surface (positive strain e) and compressing of its inner surface
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Z. L. Wang/Nanopiezotronics
a ε
z
c
d
ε>0
Ez>0
Vp>0 (V+
p)
ε=0
Ez=0
Vp=0
ε<0
Ez<0
Vp<0 (Vp- )
b
F
z
Ez
e
I
0
f
I
Vm 0
RL V p-
Vp
FET
VG
+ + + +
-
-
Silicon
- Gate oxide
VSD
h
PE-FET
+ + + +
-
-
-
Silicon
-
V
V
F
2. Results
-
- - - - -p -
- + + + + + +
Vp+
2.1. Nanogenerators
Trapped carriers
i PE-diode
VG
Vp
+
VSD
-
+
Vp
VL
ZnO
g
Vm
-
+
PZ-field direction is closely parallel to the z-axis (NB direction) at the tensile surface and antiparallel to the z-axis at the
compressed surface (Fig. 2c). Under the first-order approximation, as a result of flipping of the PZ field across the width
of the NB, the electric potential distribution from the compressed to the stretched side surface is approximately between
Vp– (negative) to V+p (positive). The electrode at the base of the
NB is grounded. Note V+p and Vp– are the voltages produced by
the PZ effect. The potential is created by the relative displacement of the Zn2+ cations with respect to the O2– anions
because of the PZ effect in the wurtzite crystal structure
(see Fig. 1); thus, these ionic charges cannot freely move and
cannot recombine without releasing the strain (Fig. 2d). The
potential difference is maintained as long as the deformation
is in place and no foreign free charges (such as from the metal
contacts) are injected. The process shown in Figure 2a–d is
the fundamental principle of piezotronics for creating functional devices such as the nanogenerator, piezoelectric transistor, and piezoelectric diode.
V
-
p
- - - - - -
- + + + + + +
Vp+
V
F
Figure 2. The physical principle of nanopiezotronics. a–d) The principle
of the piezoelectric nanogenerator [3]: a) schematic definition of a nanobelt (NB) and the coordination system; b) longitudinal strain ez distribution in the NB after being deflected by an atomic force microscope
(AFM) tip from the side; c) the corresponding longitudinal piezoelectricinduced electric field Ez distribution in the NB; d) potential distribution
in the NB as a result of the piezoelectric effect; e) metal–semiconductor
Schottky contact between the AFM tip and the semiconductor ZnO NB
at reverse bias, which is responsible for separating and preserving the
piezoelectric charges; f) metal–semiconductor Schottky contact between
the AFM tip and the semiconductor ZnO NB at forward bias, which is
responsible for releasing the accumulated piezoelectric charges.
g) Schematics of conventional field-effect transistors (FETs) using a single nanowire/nanobelt, with gate, source, and drain. h) The principle of
the piezoelectric field-effect transistor (PE-FET) [4], in which the piezoelectric potential across the nanowire created by the bending force F replaces the gate in a conventional FET. The contacts at both ends are
Ohmic. i) The principle of the piezoelectric gated diode [6], in which one
end is fixed and enclosed by a metal electrode, and the other end is bent
by a moving metal tip. Both have Ohmic contact with ZnO. The piezoelectric potential V+p at the tensile surface acts like the p–n junction in a
conventional diode.
(negative strain e; Fig. 2b). An electric field Ez along the NB
(z-direction) is then created inside the NB through the PZ
effect, Ez = ez/d, where d is the PZ coefficient[3] along the NB
direction that is normally the positive c-axis of ZnO. The
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The physical principle for creating and separating the PZ
charges in the NB (see Fig. 2a–d) is the first half of a nanogenerator[4] for converting mechanical energy into electricity.
Then, the preserving and discharging is the second half of the
nanogenerator, which is a coupling process of piezoelectric
and semiconducting properties.
We now consider the discharge process. In the first step, the
atomic force microscope (AFM) conductive tip that induces
the deformation force F is in contact with the stretched
surface of positive potential V+p (Fig. 2e). The metal tip has a
potential of nearly zero, Vm = 0, so the metal tip/ZnO interface is negatively biased for DV = Vm – V+p < 0. Considering the
n-type semiconductor characteristic of the as-synthesized
ZnO NBs, the metal/ZnO semiconductor (M/S) interface in
this case is a reverse-biased Schottky diode (Fig. 2e), and little
current flows across the interface. This is the process of preserving and accumulating charges.
In the second step, when the AFM tip is in contact with the
compressed side of the NB (Fig. 2f), the metal tip/ZnO interface is positively biased for DV = VL = Vm – Vp– > 0. The M/S interface in this case is a positively biased Schottky diode, and it
produces a sudden increase in the output electric current. The
current is the result of DV driven flow of electrons from the
semiconductor ZnO NB to the metal tip. The flow of the free
electrons from the loop through the NB to the tip will neutralize the ionic charges distributed in the volume of the NB and
thus reduce the magnitude of the potentials Vp– and V+p. This is
the process of outputting piezoelectric charges. The principle
demonstrated in Figure 2 is the fundamental of nanopiezotronics.
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2007, 19, 889–892
Z. L. Wang/Nanopiezotronics
Field-effect transistors (FETs) based on nanowires/nanotubes
are one of the most studied nanodevices. A typical NW FET is
composed of a semiconducting NW that is connected by two
electrodes at the ends and placed on a silicon substrate covered
by a thin layer of gate oxide. A third electrode is built between
the NW and the gate oxide (Fig. 2g). The electric signal output
from the drain electrode of the NW is controlled by a gate voltage applied between the gate and the NW. An NW-based sensor
is a source–drain structured NW FETwithout a gate, thus, a large
portion of the NW is exposed to the environment. The mechanism of NW sensors for sensing gases, biomolecules, or even
viruses relies on the creation of a charge depletion zone in the
semiconductor NW by the surface adsorbed sensing targets.[5]
By connecting a ZnO NW across two electrodes that can
apply a bending force to the NW, the electric field created by
piezoelectricity across the bent NW serves as the gate for controlling the electric current flowing through the NW
(Fig. 2h).[5] Once deformed by an external force F, a piezoelectric potential is built across the bent NW, and some free
electrons in the n-type ZnO NW may be trapped at the surface of the positive side and become nonmovable charges,
thus, lowering the effective carrier density in the NW. On the
other hand, even though the positive-potential side could be
partially neutralized by the trapped electrons, the negative
potential side remains unchanged. The free electrons will be
repulsed away by the negative potential and leave a charge
depletion zone around the compressed side surface. Consequently, the width of the conducting channel in the ZnO NW
becomes smaller and smaller while the depletion region becomes larger and larger with the increase of the NW bending.
Therefore, the role played by the piezoelectric-induced field
across the width of the NW (Fig. 2h) is analogous to the case
of applying a gate voltage across the width of the ZnO NW as
for a typical NW FET (Fig. 2g). The process shown in Figure 2h gives a good explanation of the transport property of a
ZnO NW as observed by Lin et al.[6]
2.3. Piezoelectric Nanosensors
As illustrated in the last section, the PE-FET can be turned
“on” or “off” by applying mechanical force/pressure to bend a
NW/NB. A nanometer-sized force/pressure sensor has been
demonstrated for measuring forces in the nanonewton range
and even smaller.[7] Since the bending curvature of the NW is
directly related to the force applied to it, the mechanical force
can be retrieved from its bending shape, which is the principle
of the force sensor. Based on the principle shown in Figure 2,
a zero-power sensor can also be designed.
2.4. Piezoelectric-Gated Diodes
A piezoelectric-gated diode[8] was demonstrated using a
two-probe technique.[6] The difference between a PE diode
Adv. Mater. 2007, 19, 889–892
from a PE-FET is that only one side of the bent NB is in contact with the probe. One probe holds the stationary side of the
NB, and the other probe bends the NB from the tensile side
(Fig. 2i).
The observed diode effect may be explained using the
mechanism demonstrated in Figure 2a–d. When the probe
pushes a NB and bends it, a positive potential is produced at
the stretched side of the NB due to the piezoelectric effect.
As a result, a potential barrier V+p is produced at the interface
between the tip and the NB with the NB being at the higher
potential. Such an energy barrier may effectively serve as a
p–n junction barrier at the interface that resists the current
flow from the tip to the NB, but current can flow from the NB
to the tip. The magnitude of the barrier increases with the
increase of the degree of NB bending, resulting in a drastic
increase of the rectifying effect. This is a simple piezoelectric
field gated diode.[6]
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2.2. Piezoelectric Field-Effect Transistors
3. Perspectives
Based on the semiconducting and piezoelectric coupled
properties, several important prototype devices, such as nanogenerators, PE-FETs, piezoelectric diodes, and piezoforce/
pressure sensors, have been demonstrated. These devices are
the fundamental components of nanopiezotronics.
Piezotronics based on nanowires/nanobelts as the fundamental building blocks have the following unique advantages:
First, the NW-based nanogenerators can be subjected to extremely large deformation, so they can be used for flexible
electronics as a flexible/foldable power source. Secondly, the
large degree of deformation that can be withstood by the
NWs is likely to result in a larger volume density of power
output. Third, ZnO is a biocompatible and biosafe material;[9]
it has great potential as an implantable power source within
the human body. Fourth, the flexibility of the polymer substrate used for growing ZnO NWs/NBs makes it feasible to
accommodate the flexibility of human muscles so that we can
use the mechanical energy (body movement, muscle stretching) in the human body to generate electricity.[10] Fifth, ZnO
NW/NB nanogenerators can directly produce current as a
result of their enhanced conductivity with the presence of
oxygen vacancies. Finally, ZnO is an environmentally “green”
material. The phenomena we have demonstrated for ZnO can
also be applied to other wurtzite-structured materials, such as
GaN and ZnS.
The future in nanotechnology research lies in the area from
single devices, to arrays of devices with multifunctionality, to
an integrated nanosystem. It is important to find various approaches that are feasible for harvesting energy and recycling
energy from the environment to self-power a nanosystem so
that it can operate wirelessly, remotely, and independently
with a sustainable energy supply. It is also important to develop zero-power sensors that respond to a change in the environment. The principle demonstrated for the piezoelectric
nanogenerator could be the foundation for self-powered
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nanosystems. It also has the potential to harvest/recycle energy from the environment and/or recycle energy that is wasted,
such as the energy when walking.
The piezoelectric FETs and diodes are outstanding examples of devices made using piezoelectric NWs and NBs. Based
on the electromechanical coupled properties of the nanostructures, novel and unique applications will be explored in areas
of sensors, actuators, switches, and microelectrochemical systems (MEMS).
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Adv. Mater. 2007, 19, 889–892